1. Field of the Invention
The invention relates generally to a circuit configuration and method of manufacture of a transient voltage suppressor (TVS). More particularly, this invention relates to an improved circuit configuration and method of manufacture with simplified configuration to integrate steering diode with the Zener diode further reduce the capacitance for a transient voltage suppressor (TVS).
2. Description of the Relevant Art
The device configurations and method of manufacturing the transient voltage suppression (TVS) device are still confronted with the technical challenges to further reduce the capacitance while maintaining a simplified and low cost manufacturing process and shrinking the area occupied by the TVS chips. Specifically, the transient voltage suppressors (TVS) are commonly applied for protecting integrated circuits from damages due to the inadvertent occurrence of an over voltage imposed onto the integrated circuit. An integrated circuit is designed to operate over a normal range of voltages. However, in situations such as electrostatic discharge (ESD), electrical fast transients and lightning, an unexpected and an uncontrollable high voltage may accidentally strike onto the circuit. The TVS devices are required to serve the protection functions to circumvent the damages that are likely to occur to the integrated circuits when such over voltage conditions occur. As increasing number of devices are implemented with the integrated circuits that are vulnerable to over voltage damages, demands for TVS protection are also increased. Exemplary applications of TVS can be found in the USB power and data line protection, Digital video interface, high speed Ethernet, Notebook computers, monitors and flat panel displays.
FIG. 1A-1 shows a conventional TVS circuit implemented with diode array commonly applied for electrostatic discharge (ESD) protection of high bandwidth data buses. The TVS array includes a main Zener diode operated with a pair of steering diodes, i.e., the high side steering diode and the low side steering diode. The high side steering diode connects to the voltage source Vcc and the low side steering diode connects to the ground terminal GND with an input/output port connected between the high side and low side steering diodes. The Zener diode has a large size to function as an avalanche diode from the high voltage terminal, i.e., terminal Vcc, to the ground voltage terminal, i.e., terminal Gnd. At a time when a positive voltage strikes on one of the I/O (input/output) terminal, the high side diodes provide a forward bias and are clamped by the large Vcc-Gnd diodes, e.g., the Zener diode. The high side and low side steering diodes are designed with a small size to reduce the I/O capacitance and thereby reducing the insertion loss in high-speed lines such as fast Ethernet applications.
As an industry trend, the steering diodes are integrated with the Zener diodes. FIGS. 1A-2 and 1A-3 are two diagrams to show the integration of the steering diodes with the Zener diode. The high side and low side terminals are not visible from the outside. FIG. 1A-2 shows the integration of the high side steering diodes and low side steering diodes with a uni-directional Zener diode. To the outside, the diode unit looks like a Zener diode with very low capacitance, but internally high-side and low-side diodes are integrated with a Zener diode. The internal circuit is the same as the circuit in FIG. 1A-1. The I/O terminal is the cathode, and the GND terminal is the anode, and the VCC terminal may be internalized so that it is not noticed from the outside. FIG. 1A-3 shows the integration of the high side steering diodes and low side steering diodes with a bi-directional Zener diode circuit. However, for modern application to the electronic devices, the protection circuit implemented with such integration must be carried out without increasing the layout areas. Furthermore, they must have careful design optimization to obtain the best tradeoff between the capacitance and the forward biased voltage of the steering diodes in order to achieve a good overall voltage clamping.
FIG. 1B-1 shows a standard circuit diagram for a conventional TVS circuit and FIG. 1B-2 is a cross sectional view for showing the actual implementation of the TVS circuit applying the CMOS processing technologies to provide the TVS circuit as integrated circuit (IC) chips. As shown in FIG. 1B-2, the device is manufactured using the CMOS processing technologies to produce diodes and NPN and PNP transistors in the semiconductor substrate with the diodes and the transistors extended along a lateral direction. The TVS circuits produced by implementing the device layout and configurations thus occupy greater areas on a substrate. It is therefore difficult to miniaturize the electronic device protected by TVS circuits as shown in FIG. 1B-1 and FIG. 1B-2.
The inventor of this patent application disclosed a TVS circuit in a pending patent application U.S. Ser. No. 11/606,602 with new and improved device configuration shown in FIG. 1C. FIG. 1C shows a TVS circuit implemented with a main Zener diode formed in a P Body/N-Epi junction. The TVS circuit as shown in FIG. 1C has significant improvement by reducing the areas occupied by the device because the main Zener diode and high side diodes are now formed with a vertical configuration. This circuit uses two I/O terminals and two corresponding sets of high side and low side diodes. The high side steering diodes is further insulated from the main Zener diode with isolation trenches thus preventing inadvertent turning on of the parasitic thyristor along the lateral direction.
The inventor of this patent application disclosed another TVS circuit in another pending patent application U.S. Ser. No. 12/286,817 with new and improved device configuration shown in FIG. 1D. FIG. 1D shows a TVS circuit with a pair of steering diodes integrated with a main Zener diode wherein the high-side diode, the low-side diode and the main Zener diode are all manufactured as vertical diodes in a semiconductor substrate. The high-side diode overlaps with the main Zener diode such that the TVS now occupies significantly less area to allow for further miniaturization of the electronic devices protected by the improved TVS configuration disclosed in FIG. 1D. Optionally, the VCC metal may be left out altogether. This device employs source sinker regions to define the area of the high side diode. However, the device as shown in FIG. 1D may experience difficulties in controlling the uniformity of dopant profiles across the entire wafer to maintain consistent and accurately controllable device performance characteristics among multiple TVS chips produced from a single batch applying same manufacturing processes. Additionally, in order to further improve the device performance, there are still demands to further reduce the capacitances of the steering diodes.
Therefore, a need still exists to simplify the device configuration such that the TVS devices can be manufactured with more controllable and uniform doping profiles and device characteristics. Also, there are additional requirements to further reduce the capacitance of the steering diodes. In order to achieve these purposes, there are demands to provide new and improved device configurations with new structural layout and manufacturing method to achieve these goals.